Elaborate techniques have been developed for quality control in the semiconductor fabrication industry. One widely used technique is the use of test wafers in the production line. The test wafers are subjected to the same processes as the integrated circuit wafers in the production line, but the test wafers have special patterns that are particularly suited for detecting variations in the processing that can produce defects and affect quality or function. A typical fabrication facility (also referred to as a “fab”) starts between 0.1 and 3 test wafers for every production wafer, with an average number being about 0.8 test wafer per production wafer.
Test wafers are expensive; some cost more than $100. A fab that processes about 150,000 production wafers per year could spend between $7 Million and $20 Million per year just for the test wafers. Because of the large cost of test wafers, it is common practice to reuse, recycle and reclaim test wafers as often as possible. Wafers are reused by processing a wafer in a second pass through the same tool, or through a second tool. Wafers are recycled by subjecting them to a chemical cleaning process to prepare them for further use. Wafers that cannot (or can no longer) be recycled are reclaimed by polishing the surface. The reclaimed wafer is like a new wafer, except the polishing reduces its thickness. Generally speaking, a new test wafer can only be reclaimed 4 to 5 times due to minimum thickness constraint. New or reclaimed test wafers might be used for several times by recycling.
Popovich, Sandy et al., “Implementation of a Test Wafer Inventory Tracking System to Increase Efficiency in Monitor Wafer Usage,” IEEE/SEMI Advance Semiconductor Manufacturing Conference, June 1997, pp 440-443, is incorporated by reference herein. Popovich et al. describe a system that automates the wafer ordering process and forces re-use of existing wafers.
Foster, Bryce et al., “Simulation of Test Wafer Consumption in a Semiconductor Facility,” IEEE/SEMI Advance Semiconductor Manufacturing Conference, August, 1998, pp 298-302, is incorporated by reference herein. Foster et al. describe a simulation of wafer usage, in which any test wafer has a fixed probability of being suitable for re-use. Foster et al. note, “The types of test wafers downgraded between monitors are not tracked. (e.g., a Low Quality test wafer can be downgraded to a monitor/stockpile that uses High Quality test wafers).”
A given recycled test wafer cannot, however, be used for testing every possible flow. A wafer that can no longer be reused in the same tool, but can be passed to another tool for use is “downgraded.” The sequence of tools that can use a test wafer during a series of operations separated by downgrades defines a “downgrade flow.” A downgraded wafer can be used for monitoring certain tools, but not others. Some monitoring operations require a new or reclaimed wafer. Some monitoring operations require new or reclaimed wafers or wafers that have only been subjected to certain processing steps (but not others).
In addition to the downgrade flow constrains, some tools cannot be tested using a raw test wafer, but need test wafer that are first processed by certain tools. For example, film deposition process steps are necessary for a test wafer used by an etch-rate monitor in etch tools. That means, capacity must be reserved in film deposition tools for preparing etch-rate test wafers.
Thus, inventory management of test wafers is a complex matter, for which a better understanding is desired.